(1) Field of the Invention
The present invention generally relates to an evaporative fuel control apparatus, and more particularly to an evaporative fuel control apparatus of an internal combustion engine for feeding fuel vapor from a fuel tank to an intake system through a purge passage in order to introduce fuel vapor into a combustion chamber.
(2) Description of the Related Art
Conventionally, an evaporative fuel control apparatus is known in which evaporative fuel gas generated in a fuel tank is directly fed to an intake system through a purge passage, which passage directly connects the fuel tank and an intake pipe of an engine, in order to burn fuel vapor in a combustion chamber. Further, an evaporative fuel control apparatus is suggested in Japanese Laid-Open Patent Application No. 62-135625, which includes detecting means for detecting an amount of evaporative fuel gas and controling means for decreasing an amount of fuel injected by a fuel injection valve in response to a detected amount of evaporative fuel gas. This evaporative fuel control apparatus detects an amount of evaporative fuel gas based on an interrelation between temperatures of cooling water, intake air, and fuel. A correction factor, for decreasing an amount of injected fuel at start-up of an engine, is calculated from the amount of detected fuel vapor. This is because when starting an engine, an air-fuel ratio is extremely fuel rich resulting in undesired conditions at start-up of the engine. The control apparatus is comprised so as to appropriately control, by means of a correction factor, an amount of injected fuel at start-up of the engine, thus realizing a good start of the engine.
In the above mentioned conventional evaporative fuel control system, the coefficient of correction for decreasing an amount of injected fuel at start-up of an engine, calculated from the amount of fuel vapor, is simply calculated on the basis of an interrelation between temperature of cooling water, intake air, and fuel. Generally, evaporative fuel gas generated in a fuel tank comprises a mixture of pure fuel (gasoline) vapor and air in the fuel tank. Increased fuel vapor in the evaporative fuel gas makes an air-fuel ratio in the rich side, on the other hand, the air in evaporative fuel gas makes an air-fuel ratio in lean side. Accordingly, an increase of air in evaporative fuel gas requires that an amount of fuel to be injected is increased, which is contrary to the correction required by the correction factor mentioned above.
The correction of an amount of fuel in the above mentioned apparatus is mainly for the starting time of an engine. Changes in temperature conditions while the engine is running are not considered. Additionally, a volume of empty space inside a fuel tank is one of the factors increasing and decreasing generated fuel vapor while engine is running. Therefore, accurate controling of the mixture can not be performed with the conventional apparatus, because the conventional apparatus does not take into account the effect of the amount of air in evaporative fuel gas.